In sewing machines, the needle thread is monitored, in general, between the endless thread reserve and the needle. The looper thread is monitored between the endless thread reserve and the shuttle in the case of multiple-thread chain stitch sewing machines, and between the shuttle and the stitch formation site in the case of double lockstitch sewing machines. After a thread disturbance appears, the thread monitor sends a signal eliciting a response from the machine.
For example, a thread monitor for monitoring the needle thread is disclosed in West German Utility Patent No. 69,13,073. To achieve this, a disk, which is set into rotation by the needle thread being pulled off from the thread reserve and is interrupted by radially extending slots in its edge zone, is monitored by a sensor device. As soon as one of the slots is flush with the sensor device, an impulse is sent to an electronic switch of a control circuit. When the disk stops after breaking of the needle thread and no more impulses are sent to the electronic switch, the sewing machine is stopped.
A thread monitor, whose sensor device, which serves to monitor the needle thread, is held on a holder fastened on the head of the sewing machine, is disclosed in U.S. Pat. No. 4,754,722.
The needle thread is led through the monitoring zone of the sensor device to the needle. As a consequence of the upward and downward movements of the needle bar during sewing, the needle thread is swung out of its middle position alternatingly in opposite directions, so that it performs an oscillating motion at right angles to the direction of thread pull and it traverses the monitored zone of the sensor device once per stitch, oscillating to and fro. Disappearance of this thread movement is indicated as a disturbance.